This invention generally relates to isotopic separation, and is specifically concerned with a system and method for continuously separating zirconium isotopes by a combination of balanced ion migration and chromatography.
The use of zirconium for forming fuel rod cladding for nuclear fuels is well known in the prior art. In nature, zirconium exists as a mixture of isotopes which includes zirconium 90, zirconium 91, zirconium 92, zirconium 94 and zirconium 96. Of all these isotopes, zirconium 91 is the least desirable to use in such fuel rod cladding since its relatively high thermal neutron cross-section causes it to absorb thermal neutrons and thereby to impede the uranium fission reaction which is desirable in an operational fuel rod assembly. In naturally occurring zirconium, zirconium 91 constitutes only about 11% of the overall weight of the metal, the balance being constituted by zirconium 90 (51.5%), zirconium 92 (17%), zirconium 94 (17.5%) and zirconium 96 (3.0%). However, because the thermal neutron cross-section of zirconium 91 is 158 times that of zirconium 90, 6 times more than that of zirconium 92, 16 times more than that of zirconium 96 and 18 times that of zirconium 94, the 11% by weight component of zirconium 91 in naturally occurring zirconium counts for 73% of the total thermal neutron cross-section of such zirconium.
The fact that zirconium 91 accounts for almost three quarters of the entire thermal neutron cross-section of naturally occurring zirconium has motivated the development of various isotopic separation techniques designed to get rid of or at least reduce the amount of zirconium 91 in zirconium. In one such technique, a compound of zirconium is vaporized and exposed to a pulse of light generated by a CO.sub.2 laser tuned to the vibrations of the bond of either zirconium 90 or 91 and the other constituent atoms joined to the zirconium. The tuned pulses of light cause these bonds to resonate and to break, thus liberating either zirconium 90 or zirconium 91, depending upon the chosen frequency of the light.
While such laser-induced isotopic separation has proven to be effective for its intended purpose, it is unfortunately expensive and capable of separating only relatively small amounts of zirconium isotopes at any given time. Hence it does not lend itself to a scaled-up, bulk-separation process that is capable of inexpensively providing the large quantities of zirconium 91-depleted zirconium needed every year for the fabrication of new fuel assemblies and fuel containers.
Other methods are known which employ electrolytic forces to separate isotopes of other elements, such as potassium. In this technique, ions of naturally occurring potassium are introduced into an electrolyte, which may be an aqueous solution of HCl. The electrolyte and dissolved zirconium ions are introduced into a column filled with an inert particulate material which provides a lengthened tortuous flowpath for the zirconium ions to travel through, and an electric potential is applied across the column. The voltage of this potential attracts potassium ions and hydrogen ions toward the cathode, while simultaneously creating a counter-flow of chlorine ions toward the anode. The voltage is strong enough so that sufficient electrolytic force is applied to the lighter potassium ions to cause a net migration of such ions toward the cathode, but is yet not so strong as to apply such a net migration movement of the heavier ions toward the cathode. Because potassium 41 ions are approximately 5% heavier than potassium 39 ions, they are not as mobile in the liquid medium of the electrolyte, and the electrolytic force applied to them by the cathode is insufficient to overcome the forces of kinetic agitation which causes them to move randomly about the electrolyte in Brownian fashion, and the counter-flow of non-potassium negative ions flowing toward the anode. This combination of forces causes these heavier ions to migrate toward the anode. Because of the balance between the flow of potassium 39 ions toward the cathode and counter-flow of potassium 41 ions toward the anode, no net flow of potassium ions occurs in the electrolyte. Eventually, over a period of time, the region of the electrolyte in the vicinity of the cathode will become enriched in potassium 39, while the region of the electrolyte in the vicinity of the anode will become enriched in potassium 41.
Unfortunately, while the technique of separating isotopes by balanced ion migration has the potential of inexpensively separating bulk amounts of such isotopes, its effectiveness in separating such bulk amounts has thus far been limited by a number of factors. For example, while the weight difference between potassium 41 ions and potassium 39 ions is approximately 5%, the weight differences between zirconium 90 (which constitutes a little over 50% of all naturally occurring zirconium) and zirconium 91 is only about 1%. Hence, the balance that must be struck between the electromigratory forces and the kinetic agitation forces are more difficult to attain and maintain. Additionally, longer migration times through longer column lengths are necessary to achieve the same degree of separation with zirconium isotopes that was achieved with potassium isotopes. Still another limitation associated with the adaptation of prior art potassium isotope separation techniques to zirconium isotope separation stems from the single-column type of device used in the prior art. Such a single column affords batch processing; it does not, by itself, provide the kind of continuous zirconium isotope separation that would be required for practical, large scale production of zirconium that is deficient in zirconium 91. The problem of adapting prior art techniques and methods is further confounded by the fact that, in the case of zirconium, we are attempting to remove the second-lightest of four isotopes, instead of either the lightest or the heaviest of these isotopes from the others. The necessity of removing one of the middle weight isotopes from the others instead of only the lightest or the heaviest isotope again necessitates longer separation times, as well as the maintenance of delicate balances between the electric potential used to create an electromigratory force, and the forces of thermal agitation in the electrolyte.
Clearly, what is needed is a system and method for separating isotopes of an element whose weight differences are only about 1% in a continuous fashion. Ideally such a system and method would be able to effectively remove all or at least most of the zirconium 91 from the balance of zirconium isotopes which occur in naturally occurring zirconium in an economical fashion by means of a system which would be relatively simple and inexpensive to construct and to operate. Such a system and method should be fast in operation, so that large quantities of zirconium which is deficient in zirconium 91 could be produced in short amounts of time. Finally, it would be desirable if the operation of the system and method could not easily be disturbed by either external shock or changes in the ambient temperature.